Incipient conditions of cohesive sediments in oscillatory flows

Abstract

In this study, the incipient conditions of cohesive sediments with different sizes and water content were experimentally investigated in an oscillatory U-shaped tunnel. A total of 100 experimental runs were conducted with different sediment samples of different yield stresses. The relationship between the incipient condition and the yield stress of bed sediments in oscillatory flows was determined. Further, the effect of yield stress on the incipient velocity of bed sediments and the critical bed shear stress in oscillatory flow were analyzed. The experimental results indicate that the critical Shields parameter of bed sediments in oscillatory flow is exponentially related to the yield stress, which is similar to that in a unidirectional flow. When the bed sediment size is less than 65μm, the incipient condition of cohesive sediment in oscillatory flow is primarily affected by the yield stress; when the sediment size is greater than 500μm, the effect of yield stress on the incipient condition of bed sediment in oscillatory flow is negligible; in addition, when 40.65μm≤D50≤500μm, the bed sediments in oscillatory flow are easier to be transported than in the unidirectional flow in the case where bed sediments exhibited the same yield stress. When D50≤40.65μm, the cohesive sediments in oscillatory flow are more difficult to be transported than that in a unidirectional flow, for cases where cohesive sediments have the same yield stress. Finally, an empirical formula to calculate the incipient condition of the bed sediment with different fluidization levels in oscillatory flows is proposed by introducing the yield stress.